Alda was born Alphonso Joseph D'Abruzzo on January 28, 1936, in New York City, and had a peripatetic childhood as his parents traveled around the United States in support of his father's job as a performer in burlesque theatres.[4] His father, Robert Alda (born Alphonso Giuseppe Giovanni Roberto D'Abruzzo), was an actor and singer, and his mother, Joan Browne, a homemaker and former beauty pageant winner.[5] His father was of Italian descent and his mother was of Irish ancestry.[6] His adopted surname, "Alda," is a portmanteau of ALphonso and D'Abruzzo.

During Alda's young junior year, he studied in Paris, acted in a play in Rome, and performed with his father on television in Amsterdam. In college, he was a member of the ROTC, and after graduation, he served for a year at Fort Benning, and then six months in the U.S. Army Reserve.[9][10] A year after graduation, he married Arlene Weiss, with whom he has three daughters: Eve, Elizabeth, and Beatrice. Two of his eight grandchildren are aspiring actors. The Aldas have been longtime residents of Leonia, New Jersey.[11] Alda frequented Sol & Sol Deli on Palisade Avenue in the nearby town of Englewood, New Jersey—a fact mirrored in his character's daydream about eating whitefish from the establishment, in an episode of M*A*S*H in which Hawkeye sustains a head injury.[12]

In early 1972, Alda auditioned for and was selected to play the role of "Hawkeye Pierce" in the TV adaptation of the 1970 film MASH.[8] He was nominated for 21 Emmy Awards, and won five. He took part in writing 19 episodes, including the finale, and directed 32, including the finale. When he won his first Emmy Award for writing, he was so happy that he performed a cartwheel before running up to the stage to accept the award. He was also the first person to win Emmy Awards for acting, writing, and directing for the same series.[citation needed]Richard Hooker, who wrote the novel on which M*A*S*H was based, did not like Alda's portrayal of Hawkeye Pierce (Hooker, a Republican, had based Hawkeye on himself, whereas Alda and the show's writers took the character in a more liberal direction).[citation needed] Alda also directed the show's 1983 2½-hour series finale "Goodbye, Farewell and Amen", which remains the single most-watched episode of any television series.[8] Alda is the only series regular to appear in all 251 episodes.[17]

Alda commuted from Los Angeles to his home in New Jersey every weekend for 11 years while starring in M*A*S*H.[18] His wife and daughters lived in New Jersey, and he did not want to move his family to Los Angeles, especially because he did not know how long the show would last.

Alan Alda, father Robert Alda, and half-brother Antony Alda appeared together in an episode of M*A*S*H, "Lend a Hand", during season 8. Robert had previously appeared in "The Consultant" in season 3.

During the first five seasons of the series, the tone of M*A*S*H was largely that of a traditional "service comedy", in the vein of shows like McHale's Navy. However, as the original writers gradually left the series, Alda gained increasing control, and by the final seasons had become a producer and creative consultant. Under his watch, M*A*S*H retained its comedic foundation, but gradually assumed a somewhat more serious tone, openly addressing political issues. As a result, the 11 years of M*A*S*H are generally split into two eras: the Larry Gelbart/Gene Reynolds "comedy" years (1972–1979), and the Alan Alda "dramatic" years (1979–1983).

For the first three seasons, Alda and his co-stars Wayne Rogers and McLean Stevenson worked well together, but later, tensions increased, particularly as Alda's role grew in popularity. Rogers and Stevenson both left the show at the end of the third season.[19] At the beginning of the fourth season, Alda and the producers decided to find a replacement actor to play the surrogate parent role formerly taken by Colonel Blake. They eventually found veteran actor and fan of the series, Harry Morgan, who would star as Colonel Sherman T. Potter, becoming another of the show's protagonists.[20]Mike Farrell was also introduced as Alda's co-star BJ Hunnicutt.

In his 1981 autobiography, Jackie Cooper (who directed several early episodes) wrote that Alda concealed a lot of hostility beneath the surface, and that the two of them barely spoke to each other by the time Cooper’s directing of M*A*S*H ended.[21]

During M*A*S*H's run and continuing through the 1980s, Alda embarked on a successful career as a writer and director, with the ensemble dramedyThe Four Seasons being perhaps his most notable hit. Betsy's Wedding is his last directing credit to date. After M*A*S*H, Alda took on a series of roles that either parodied or directly contradicted his "nice guy" image.[8] His role as a pompous celebrity television producer in Crimes and Misdemeanors was widely seen as a self-parody, although Alda has denied this.

In 1993, he co-starred with Woody Allen (also the director), Diane Keaton, and Anjelica Huston in the comedy/mystery Manhattan Murder Mystery. The four play a quartet of amateur crime solvers who become entangled in a murder plot possibly perpetrated by Keaton and Allen's neighbor. Alda's character is Ted, a playwright secretly in love with Keaton's character Carol, but who eventually falls for Huston's character Marcia.

Beginning in 2004, Alda was a regular cast member on the NBC program The West Wing, portraying RepublicanU.S. Senator and presidential candidate Arnold Vinick, until the show's conclusion in May 2006. He made his premiere in the sixth season's eighth episode, "In The Room", and was added to the opening credits with the 13th episode, "King Corn." In August 2006, Alda won an Emmy for his portrayal of Arnold Vinick in the final season of The West Wing. Alda had been a serious candidate, along with Sidney Poitier, for the role of President Josiah Bartlet before Martin Sheen was ultimately cast in the role.[citation needed]

Alda starred in the original Broadway production of the play 'Art', which opened on March 1, 1998, at the Bernard B. Jacobs Theatre. The play won the Tony Award for best original play.

Alda also had a part in the 2000 romantic comedy What Women Want, as the CEO of the advertising firm where the main characters worked.

In the spring of 2005, Alda starred as Shelly Levene in the Tony Award-winning Broadway revival of David Mamet's Glengarry Glen Ross, for which he received a Tony Award nomination for Best Featured Actor in a Play. Throughout 2009 and 2010, he appeared in three episodes of 30 Rock as Milton Greene, the biological father of Jack Donaghy (Alec Baldwin). In January 2010, Alda hosted The Human Spark, a three-part series originally broadcast on PBS discussing the nature of human uniqueness and recent studies on the human brain.

In 2011, Alda was scheduled to guest star on Law & Order: LA, portraying former police and naval officer John Winters, the father of the former main character Rex Winters. It is unknown whether he filmed his role before the series was redesigned and Rex Winters written off.

After the release of the movie Tower Heist, Alda was devastated when on December 7, 2011, he lost his idol and decades-long friend Harry Morgan from M*A*S*H. Upon Morgan's death, Alda released a statement referring to a M*A*S*H reunion event two years before: "We had just a wonderful time reminiscing. That was the last time I saw Harry."[26]

Alda has done extensive charity work. He helped narrate a 2005 St. Jude's Children's Hospital produced one-hour special TV show Fighting for Life.[28] He and his wife, Arlene, are also close friends of Marlo Thomas, who is very active in fund-raising for the hospital her father founded. The special featured Ben Bowen as one of six patients being treated for childhood cancer at Saint Jude. Alda and Marlo Thomas had also worked together in the early 70s on a critically acclaimed children's album entitled Free to Be You and Me, which featured Alda, Thomas and a number of other well-known character actors. This project remains one of the earliest public signs of his support of women's rights.

In 2005, Alda published his first round of memoirs, Never Have Your Dog Stuffed: and Other Things I've Learned.[18] Among other stories, he recalls his intestines becoming strangulated while on location in La Serena, Chile for his PBS show Scientific American Frontiers, during which he mildly surprised a young doctor with his understanding of medical procedures, which he had learned from M*A*S*H. He also talks about his mother's battle with schizophrenia. The title comes from an incident in his childhood, when Alda was distraught about his dog dying and his well-meaning father had the animal stuffed. Alda was horrified by the results, and took from this that sometimes we have to accept things as they are, rather than desperately and fruitlessly trying to change them.

In 2006, Alda contributed his voice to a part in the audio book of Max Brooks' World War Z. In this book, he voiced Arthur Sinclair Jr., the director of the United States Government's fictional "Department of Strategic Resources (DeStRes)".

His second memoir, Things I Overheard While Talking to Myself, weaves together advice from public speeches he has given with personal recollections about his life and beliefs.

After years of interviews, Alda helped inspire the creation of the Center for Communicating Science at Stony Brook University in 2009. He remains on the advisory board as of 2013.[30] He was named an Honorary Fellow by the Society for Technical Communication in 2014 for his work with the Center for Communicating Science and the annual Flame Challenge.[31] He is also on the advisory board of the Future of Life Institute.[3]

In Things I Overheard While Talking to Myself, Alda describes how as a teen he was raised as a Roman Catholic and eventually he realized he had begun thinking like an agnostic or atheist:

For a while in my teens, I was sure I had it. It was about getting to heaven. If heaven existed and lasted forever, then a mere lifetime spent scrupulously following orders was a small investment for an infinite payoff. One day, though, I realized I was no longer a believer, and realizing that, I couldn’t go back. Not that I lost the urge to pray. Occasionally, even after I stopped believing, I might send off a quick memo to the Master of the Universe, usually on a matter needing urgent attention, like Oh, God, don’t let us crash. These were automatic expulsions of words, brief SOS messages from the base of my brain. They were similar to the short prayers that were admired by the church in my Catholic boyhood, which they called “ejaculations.” I always liked the idea that you could shorten your time in purgatory with each ejaculation; what boy wouldn’t find that a comforting idea? But my effort to keep the plane in the air by talking to God didn’t mean I suddenly was overcome with belief, only that I was scared. Whether I’d wake up in heaven someday or not, whatever meaning I found would have to occur first on this end of eternity.

Speaking further on agnosticism, Alda goes on to say:

I still don't like the word agnostic. It's too fancy. I'm simply not a believer. But, as simple as this notion is, it confuses some people. Someone wrote a Wikipedia entry about me, identifying me as an atheist because I'd said in a book I wrote that I wasn't a believer. I guess in a world uncomfortable with uncertainty, an unbeliever must be an atheist, and possibly an infidel. This gets us back to that most pressing of human questions: why do people worry so much about other people's holding beliefs other than their own?

Alda made these comments in an interview for the 2008 question section of the Edge Foundation website.[32]

His most famous character, Dr. Benjamin "Hawkeye" Pierce on M*A*S*H, was reputed to be agnostic, but he often was shown attending Sunday services and maintained a close friendship with the camp's chaplain, Fr. Francis Mulcahy. A tenth-season episode depicted Hawkeye serving at the front and fearfully writing his will, where he left five cents to the Father, explaining, "I know you are a spiritual man and material things mean little to you, but I leave you one other thing: my everlasting respect."

^Alan Alda, interviewed by Jian Ghomeshi, CBC Radio, March 28, 2013. In response to Ghomeshi's comment that Alda had grown up in the Bronx, Alda said, "No I didn't but I can tell you're a Wikipedia reader."

^Kingergan, Ashley (Sep 27, 2010). "Noted Englewood deli closes after 60-plus years". The Record. Retrieved September 27, 2010. Perhaps the greatest tribute to the deli came from the 1970s television show M*A*S*H. Hawkeye, one of the main characters in M*A*S*H*, daydreams about whitefish from Sol & Sol after sustaining a head injury.